C-shaped clamps are generally used by workers for clamping two parts of a workpiece, or for fixing a workpiece on a work table for a processing machine in order to solder, drill, lathe, grind and mill the workpiece. A conventional C-shaped clamp generally includes a C-shaped main body and a screw which can be adjusted forward and backward, the front end of the screw possessing a movable top block. In use, a space between the top block and surface of inner side corresponding to C-shaped main body is used for receiving the workpiece. Then the screw is screwed to fix and clamp the workpiece. For workpieces whose shapes are not completely regular and planar, even if the front end of the screw of the conventional C-shaped clamp possesses a movable top block, it still cannot meet conventional C-shaped clamp multi-use requirements.